When a collimated optical beam, such as a laser beam, is transmitted through an articulated beam manipulating or delivery device, such as a robot, the beam typically goes through several reflections. Reflecting mirrors are utilized in what is known as an optical joint, to pass the beam through the articulating joints of the device. The beam travels in straight line segments between each successive optical joint.
Any error in the alignment of the mirrors in the optical joints results in progressive deviation of the point of incidence of the beam on the following mirrors. Considerable deviations render the articulating device ineffective if the mirrors are not properly aligned. Large beam travel distances magnify the mirror alignment errors and require higher accuracy in alignment.
In order to attain higher accuracy alignments of mirrors in optical joints, a light beam is conventionally shined on the mirror along one of the axes of the optical joint and is intercepted at a distance along the other axis of the optical joint, as illustrated in FIGS. 1a and 1b. A grid is used to identify the axis "0" of the optical joint at the point of incidence "A" of the reflected beam. When the optical joints and the grid are rotated 360.degree. relative to the misaligned mirror, the point of beam incidence on the grid describes a full circle of radius "OA" where OA is equal to e.times.D (e equals the angle of misalignment in radians and D is equal to the beam travel distance from the misaligned mirror to the grid surface).
Typically, the mirror orientation is corrected by adjusting screws to make the point of incidence coincide with the joint axis. To attain desired accuracy levels, the grid must be located at relatively long distances to magnify the error and allow finer adjustments. Such long distances make the calibration apparatus impractically long.
For the alignment of the final mirror in an optical chain, the alignment apparatus may be rotated about the final optical axis. Circular movement of the point of beam incidence on the grid indicates misalignment of the final mirror and can be corrected for by adjustment screws.
Referring now to FIG. 2 there is illustrated another prior art method for correcting for these shortcomings by using multiple reflecting mirrors which are pre-aligned and precisely oriented. These mirrors fold the long-beam travel distances into a compact package. However, this method has several shortcomings such as: (1) the folding mirrors add weight and expense to the apparatus; (2) the cost is increased because of the precise requirements of the folding mirrors and the need to accurately calibrate and align the apparatus itself; (3) the method does not provide quantitative measurements of the alignment error to the operator; (4) the method does not give direct feedback to the operator since the location of the apparatus may not always be within his visual range; (5) the resolution of the error is severely limited by the size of the apparatus, its weight and the limit of visual detection; and (6) the apparatus only magnifies angular misalignments.
Another common misalignment problem appears as a parallel offset in the path of the beam. This is illustrated in FIG. 3 wherein deviation of the beam off the center of the optical joint is illustrated. Because the deviation remains parallel to the optical joint center line, the deviation is not magnified by the folding mirrors. This error is usually corrected by the use of a pinhole aperture at the entrance of the beam and thereafter making the necessary required adjustments to the incoming beam source to affect beam entry through that aperture. The level of accuracy obtained, however, is limited by the size of the aperture and the limits of human perception of beam location.
The Hawkins et al U.S. Pat. No. 4,724,298 discloses a method for aligning a laser beam transmitter and a receiver. A low-powered visible laser is directed along the path to be followed by a high-powered laser. A viewer, such as a video camera, is used for rough alignment. A pellicle, a partially reflecting mirror, and three low-powered beam detectors of the pyroelectric type are used with the low-powered laser to align the beam transmitter with the beam receiver. One of the video cameras is mounted on a housing which is attached to a housing of the beam transmitter and is movable therewith for viewing the position and orientation of the beam receiver with respect to a reflector.